The Donetsk Metallurgical Plant has evaluated the quality of its semifinished 135 • 135 mm square sections of steels St, 3 and 20 in relation to the degree of deoxidation of the steel.It is known that cracks and seams which form at rolling temperatures are caused by reduced ductility resulting from the presence of sulfides, oxygen-bearing nonmetallic inclusions, or phases of complex composition at the grain boundaries [1 ]. The effect of oxygen-bearing inclusions on the quality of metal was judged from its degree of deoxidation, which was evaluated on the basis of the total aluminum content of the finished steel.The metal was made in 150-ton open-hearth furnaces by the scrap-ore process, deoxidized in the ladle with aluminum in the amount 1.2 kt/ton, and bottom-poured into big-end-up molds to obtain 5.7-ton ingots. The steel was poured under a heat-insulating mixture based on ash from the Kurakovsk heating and power plant. The ingots were rolled on a 900/950 mill.The quality of the metal was evaluated from the amount of metal rejected in the first conversion (tons/heat) for surface defects -mainly cracks, seams, and the presence of sand. Analysis of the resulting data made it possible to establish the following empirical relation between the level of rejection and the aluminum content of the finished steel ( where Rj is the quantity of semifinished sections rejected in the first conversion, tons; [All is the average aluminum content of the finished steel, %. An analysis of the reliability of these relations based on the number of points falling within a range that was a multiple of the standard deviation S = 4.876 tons showed that there were three regions in which the level of rejection underwent a distinct change in relation to the aluminum content of the steel: I.[All < 0.009% -region characterized by a high level of rejection; II. 0.009% < [AI] < 0.025% -region characterized by a consistently low level of rejection;III.[A1] > 0.025% -region characterized by an increase in the level of rejection. On the whole, heats falling within region I are characterized by an unstable level of rejection and a low aluminum content. The low content of aluminum is related to an increase in its loss due to oxidation during the tapping of the steel, which contains 0.06-0.10% C. The oxidation loss of aluminum exceeds 92% and 90% at aluminum consumptions of 1.1 and 1.5-1.9 kg/ton, respectively (Fig. 2).A high level of rejection is characteristic of so-called "mild" heats -heats with a low carbon content after melting. It is impossible to maintain the proper conditions during the boiling period in such heats or to maximize the removal of nonmetallic inclusions from the metal before tapping. The remaining inclusions, together with the inclusions lormed during deoxidation of the metal in the ladle and during crystallization of the ingot, reduce the ductility of the metal at rolling temperatures and promote the formation of cracks [ 1 ].The increase in rejections seen for steel with an aluminum content greater than 0.025% (zone II) is...
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